Animal shape has become sophisticated through evolution or phenotypic plasticity to adapt and survive changing environmental conditions. Sexual differences, which are sex-specific characteristics, form an important research topic that has also long fascinated biologists. Morphometrics is a traditional approach that enables the quantitative description and interpretation of variation in shape based on morphological features. In this study, we attempted to apply two morphometrics approaches (landmark-based or measurement-based) to identify sexual dimorphic traits in the carapace of the horsehair crab, Erimacrus isenbeckii (Brandt, 1848). Both approaches revealed that its carapace showed apparent sexual dimorphism, enabling the discrimination of the male and female carapace with more than 90 % accuracy. Moreover, the landmark-based approach found that carapace shape showed fluctuating asymmetry as well as directional asymmetry in both males and females. Fluctuating asymmetry consists of small random deviations from perfect bilateral symmetry and may be tightly linked to environmental changes. Our data suggest that responsiveness to environmental stressors during carapace formation might be different in female and male horsehair crabs. This study indicates that landmark-based geometric and traditional morphometrics can serve as a strong tool to distinguish female from male horsehair crabs using carapace shape. This is the first report to detail the use of morphological features to show sexual differences in horsehair crab. Morphometric parameters obtained from this study will be a useful reference dataset that can be applied for geographic comparisons of carapace shape of horsehair crab.
